1. Field of the Invention
The present invention relates to a three-dimensional memory element and a memory device which are used in the field of electronics and allow three-dimensional transfer and storage of charges.
2. Description of the Related Art
According to the LSI technology in the field of electronics in recent years, research and development have been directed toward micropatterning. However, since the technical development in micropatterning has almost reached its limit, research and development for three-dimensional integrated circuits (ICs) have been enthusiastically made so as to increase the packing density, the number of functions, and the processing speed of a memory. A technique using LB films is available as an effective technique for realizing three-dimensional ICs. The "LB film" is a general term of films formed by an LB method (Langmuir-Blodgett method) which is one of the methods of forming ultra thin organic films. According to the LB method, molecules can be regularly arranged on the molecular order, i.e., on the angstrom order under the conditions of normal temperature and normal pressure.
The principle of film formation by the LB method will be described below. Generally, an organic compound having a hydrophilic group can be developed to form a monomolecular film on a water surface. Especially an organic compound having a hydrophilic group at one terminal and a hydrophobic group at the other terminal, such as soap, forms a monomolecular film on a water surface while the hydrophilic group is soaked in the water if the hydrophilic and hydrophobic groups have equal strength. When a proper pressure is applied to such a monomolecular film while its surface pressure is kept at a constant value, and a substrate is vertically moved in a solution on which the monomolecular film is developed, a plurality of monomolecular films can be stacked on the substrate.
Attempts have been made by E. G. Wilson et al. to realize a large-capacity, compact memory by applying this LB film to a three-dimensional recording medium. The technical contents of these attempts are disclosed in U.S. Pat. No. 4,534,015, 4,627,029, and 4,813,016.
FIG. 17 shows a schematic arrangement of a three-dimensional memory element proposed by E. G. Wilson et al. A three-dimensional memory element 1 shown in FIG. 17 has a function of transferring charges and three-dimensionally accumulating charges by utilizing a tunnel hopping effect. This three-dimensional memory element comprises an organic monomolecular multilayer (LB film) 4 formed by alternately stacking charge accumulating portions 2 and insulating portions 3, and upper and lower electrodes 5 and 6 respectively formed on the upper and lower ends of the multilayer 4. An optical pulse generator 7 is arranged on the upper electrode side. The optical pulse generator 7 comprises a photon energy source 8, such as an LED, and a photon output adjusting unit 9. Reference numeral 11 denotes a voltage source; and 12, a voltage applying unit serving also as an amplifying feedback unit.
FIG. 18 shows the molecular structure of the organic monomolecular multilayer and the voltage source 11 which are extracted from the three-dimensional element 1 shown in FIG. 17. As shown in FIG. 18, the organic monomolecular multilayer 4 has a structure in which monomolecular layers 13 are stacked in a direction in which the upper and lower electrodes oppose each other. Conjugated bonds 13a of the polymer constituting the monomolecular layers 13 have a charge accumulating function, whereas other portions 13b have an insulating function.
FIGS. 19A to 19C show a detailed bonding state and chemical formula of the organic monomolecular multilayer 4. FIG. 19A shows a bonding state of a polymer in which derivative organic molecules (monomers) are polymerized. FIG. 19B shows the chemical formula of each monomer which is obtained by adding an alkyl group (CH.sub.2).sub.n having a relatively large electron affinity and a carboxyl group COOH to an organic molecule. FIG. 19C shows the molecular formula of a conjugated bond of the polymer. This conjugated bond has a large electron affinity and a charge accumulating effect.
FIG. 20 shows an electric potential of the organic monomolecular multilayer 4. The potential of the multilayer 4 has continuous upper and lower peaks 14 and 15, and hence forms a well potential array. The upper peak 14 corresponds to the alkyl and carboxyl groups in each monomolecular layer 13 shown in FIG. 19B and constitutes a potential barrier. The lower peak 15 corresponds to each conjugated bond of the polymer shown in FIG. 19C, and constitutes a charge accumulating portion in which charges are accumulated.
The three-dimensional memory element 1 having the above-described arrangement is operated in the following manner. Charges are injected in the organic "mono-molecular multilayer 4 by using the voltage applying unit 12 or the optical pulse generator 7. Injection of charges by means of the voltage applying unit 12 is performed by applying a pulse voltage between the upper and lower electrodes 5 and 6. Injection of charges by means of the optical pulse generator 7 is performed by radiating light on the upper electrode 5 to generate optical charges. The charges injected in the multilayer 4 are accumulated in a charge accumulating portion located nearest to the upper electrode 5. When a voltage is applied between the upper and lower electrodes 5 and 6, the charges accumulated in the lower peak 15 sequentially tunnel-hop the upper peaks 14 (insulation portions) and are transferred from the layer nearest to the upper electrode 4 to deeper charge accumulating portions. When application of the voltage between the upper and lower electrodes 5 and 6 is stopped while charges are injected in the organic monomolecular multilayer 4, the charges are held in the respective accumulating portions as they are, and data carried by the charges is stored. When stored data is to be read out, charges accumulated in the charge accumulating portion nearest to the lower electrode 6 are read out as a current from the upper electrode 5 by applying a voltage between the upper and lower electrodes 5 and 6. Alternatively, the lower electrode 6 is made of a light-emitting conductive material so that charges accumulated in the charge accumulating portion are injected in the lower electrode 6 to be optically read out.
FIG. 21 shows an electric potential of a memory element having a MIS structure in which an organic monomolecular multilayer 4 is sandwiched between a metal member and a semiconductor member. The hydrophilic and hydrophobic groups of a molecule constituting an LB film have different potentials with reference to electrons. For this reason, the potential of an LB film formed by stacking such molecules has a structure in which upper and lower peaks alternately appear. Such an LB film is sandwiched between a metal electrode M and a semiconductor substrate S, and an electric filed E is applied to the resultant structure. Then, the potential having continuous upper and lower peaks is inclined toward the semiconductor substrate S side. In this state, light is radiated from the metal electrode M side, and electrons are injected in the LB film. The injected electrons are sequentially transferred from a first lower peak al to a second lower peak a2, ... at intervals of an average time .tau.t.sub.n (tunnel hopping time). If the timings when electrons are injected and transferred are adjusted, two types of lower peaks can be formed, i.e., ones in which electrons are accumulated and ones in which no electrons are accumulated, thus forming a contrast of electron density. Within a finite time, the electrons are transferred while this contrast of electron density is kept unchanged. If application of the electric filed E is stopped during transfer, transfer of the electrons is stopped, and input data is stored in the LB film as a contrast of electron density.
In the memory element using the tunnel hopping phenomenon, a voltage is applied across both the ends of the LB film to cause charges to tunnel-hop all the upper peaks of a potential with an equal probability, thereby transferring the charges to the adjacent lower peaks of the potential. That is, all the layers are ON/OFF-operated synchronously. The three-dimensional memory device proposed by E. G. Wilson et al. is designed such that charges can be transferred while the charge density distribution in the LB film is limited within the monomolecular films of the LB film if a voltage to be applied during tunnel hopping exceeds a predetermined value. However, since the tunnel hopping phenomenon is a phenomenon based on the theory of probability having a fluctuation, charges spread in the transfer direction with a probability distribution. This spreading substantially corresponds to .sqroot.n, assuming that the total number of upper peaks (tunnel switch portions) 14 through which the charges are transferred is represented by n. This spreading can be represented by a Poisson distribution shown in FIG. 22 with the nth layer of the LB film being set as the center. Referring to FIG. 22, reference symbol .tau.t.sub.n denotes a tunnel hopping time; and l, a molecular length. Such a phenomenon is a so-called diffusion phenomenon. In order to realize reliable storage and suppress disturbance of data to be transferred, this diffusion must be minimized. As a method of preventing diffusion, a method of alternately stacking more than two types of films is available. Published Unexamined Japanese Patent Application Nos. 62-163364 and 62-189746 disclose memory devices to which the above-mentioned LB hetero film is applied. Although a memory device having an LB hetero film can suppress diffusion, an LB film formation process is very complicated. In addition, since an LB hetero film is formed by stacking films made of different materials, the possibility that adjacent films greatly influence each other physically and chemically is high. Therefore, film stacking of a hetero film may not be possible or the allowable range of optimal conditions may be greatly narrowed depending on a combination of film materials, thus undesirably limiting types of film materials. Moreover, since very complicated pulse waveforms must be used to transfer charges, transfer of charges is difficult to perform.
Another problem is that a charge trap which greatly influences data transfer tends to be generated in an LB film. With regard to such a charge trap, the following report was made in 1986 Fall Meeting of the Institute of Applied Physics (30 aZK4, Morimoto et al.): "Upon observation of the transient response characteristics of an output current when a pulse was applied to an LB film, it was found that disturbance of a current waveform was caused by traps". As is apparent from this report, it is considered that the phenomenon that charges expand in the transfer direction is not simply caused by diffusion but is also associated with charge traps. In order to reduce charge traps in an LB film, impurities must be removed and film defects must be reduced. However, in the existing techniques for forming LB films, it is difficult to reduce charge traps without influencing electric conduction. Especially when hetero films are to be formed, the number of charge traps tends to increase. Therefore, if charge diffusion is prevented in LB hetero films, charge traps are simultaneously increased in number.